There are innumerable industrial processes that require control of air flow in a duct. Such control is provided in part by valves. For low pressure vacuum systems, fine control is needed. Currently available valves, and valve systems including valve controllers, are inadequate in many respects.
Ideally, such valves create when open little or no restriction in the duct. However, rotatable vane-type valves, and piston-type valves each create very substantial restrictions (and thus pressure drop across the valve) even when fully open. Such is unacceptable for many applications.
In addition, such valves should operate as quickly as possible in order to prevent pressure fluctuations that can affect air flow upstream of the valve, and their operation should preferably be linear, which simplifies and speeds their control. Butterfly valves have a very non-linear relationship between position and open area, and are relatively slow to operate. These issues are particularly critical in applications that require constant, controlled air flow in an industrial process. One example of such is the fine control of air flow required in a semiconductor wafer resist coating operation; the air flow in part defines the quality of the resist coat, so the aim is to keep the air flow constant. However, perturbations in the vacuum system downstream of the resist coating chamber cause pressure fluctuations that propagate upstream to the chamber, thus disturbing the air flow. It is thus an aim in such systems to have a valve, and air control system using such valve, that creates essentially zero pressure drop when fully open, and provides for fine and nearly instantaneous control of the valve orifice size, to prevent downstream pressure perturbations from propagating upstream past the valve, and thus affect the flow of air in a resist coating chamber located upstream of the valve.